This invention relates to a heat transferable sheet or a sheet to be heat transfer printed, and more particularly to a heat transferable sheet which is used in combination with a heat transfer printing sheet wherein heat printing is carried out in accordance with image information by means of thermal heads, a laser beam, or the like.
Heretofore, a heat sensitive color-producing paper has been primarily used in order to obtain an image in accordance with image information by means of thermal heads, a laser beam, or the like. In this heat sensitive color-producing paper, a colorless or pale-colored leuco dye (at room temperature) and a developer provided on a base paper are contacted by the application of heat to obtain a developed color image. Phenolic compounds, derivatives of zinc salicylate, rosins and the like are generally used as such a developer.
However, the heat sensitive color-producing paper as described above has a serious drawback in that its color disappears when the resulting developed color image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation.
On the other hand, a heat sensitive transfer printing sheet wherein a heat-fusing wax layer having a pigment dispersed therein is provided on a base paper has been recently used. When this heat sensitive transfer printing sheet is laminated with a paper to be heat transfer printed, and then heat printing is carried out from the back of the heat sensitive transfer printing sheet, the wax layer containing the pigment is transferred onto the heat transferable paper to obtain an image. According to this printing process, an image having durability can be obtained, and a multi-color image can be obtained by using a heat sensitive transfer printing paper containing three primary color pigments and printing it many times. However, it is impossible to obtain an image having an essentially continuous gradation as in a photograph.
In recent years, there has been a growing demand for a method and means for obtaining an image like a photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand. One of such attempts provides a process wherein an image is projected onto a cathode-ray tube (CRT), and a photograph is taken with a silver salt film. However, when the silver salt film is an instant film, the running cost is high. When the silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing. An impact ribbon process and an ink jet process have been proposed as further processes. In the former, the quality of the image is inferior. In the latter, it is difficult to simply obtain an image like a photograph because an image treatment is required.
In order to overcome such drawbacks, there has been proposed a process wherein a heat transfer printing sheet provided with a layer of sublimable disperse dyes having heat transferability is used in combination with a heat transferable sheet, and wherein the sublimable disperse dye is transferred onto the heat transferable sheet while it is controlled to obtain an image having a gradation as in a photograph. According to this process, an image having continuous gradation can be obtained from a television signal by a simple treatment. Moreover, the apparatus used in this process is not complicated and therefore is attracting much attention.
One example of prior art technology close to this process is a process for dry transfer calico printing polyester fibers. In this dry transfer calico printing process, dyes such as sublimable disperse dyes are dispersed or dissolved in a solution of synthetic resin to form a coating composition, which is applied onto tissue paper or the like in the form of a pattern and dried to form a heat transfer printing sheet, which is laminated with polyester fibers constituting sheets to be heat transfer printed thereby to form a laminated structure, which is then heated to cause the disperse dye to be transferred onto the polyester fibers, whereby an image is obtained.
However, even if such a heat transfer printing sheet and a polyester fiber, heat transferable sheet are laminated and then subjected to heat printing by means of thermal heads or the like, it is impossible to obtain a developed color image having a high density. While one reason for this is that the surface of the polyester fiber fabric is not sufficiently smooth, it is thought that the main reasons are as follows.
In a conventional dry transfer calico printing process or a wet transfer calico printing process, the transfer of the sublimable dye onto the polyester fiber fabric is carried out with ample heating time. In contrast, heating by means of thermal heads or the like is ordinarily extremely short, whereby the dye is not sufficiently transferred onto the fiber fabric. In the dry transfer calico printing process, the transfer of the dye is accomplished by heating for about one minute at a temperature of 200.degree. C., whereas the heating by means of thermal heads is short, i.e., of the order of several milliseconds at a temperature of 400.degree. C.
We have carried out studies to eliminate the drawbacks described above primarily by improvement of the heat transferable sheet or the sheet to be heat transfer printed. As a result, we have made the following discoveries.
When a clay coated paper or synthetic paper is used as the heat transferable sheet, it cannot sufficiently receive the dye which is transferred from a heat transfer printing sheet, and therefore a developed color image having a high density cannot be obtained.
When a heat transferable sheet which has a receptive layer of synthetic resins having a low melting point is used, the synthetic resin layer per se may acquire a thermal adhesion property by the action of heat and pressure applied to the heat transfer printing sheet, and the heat transfer printing layer of the heat transfer printing sheet may be transferred onto the heat transferable sheet. Consequently, the clearness and definition of the resulting image is impaired.
Furthermore, when a heat transferable sheet which has a receptive layer of synthetic resin having a low glass transition temperature is used, the dye which is heat transfer printed onto the heat transferable sheet is sufficiently fixed, and thus a developed color image having a high density is temporarily obtained. However, it has been found that the dye becomes heat diffused with the elapse of time to distort the image.
On the other hand, when the receptive layer of the heat transferable sheet is formed from a synthetic resin having a high glass transition temperature, the heat diffusion described above can be prevented. However, it has been found that the fixing property of the dye transferred from the heat transfer printing sheet is poor. In the extreme case, when a soft polyvinyl chloride resin sheet containing plasticizers having an ester group such as dioctyl phthalate is used for the receptive layer of the heat transferable sheet, an excellent developed color image having a high density is once obtained immediately after heat transfer printing. However, the dye dissolves in the plasticizers to become diffused in the sheet. Consequently, when the image is allowed to stand ordinarily for about one week at room temperature, the resulting developed color image becomes extremely unclear, and it is virtually impossible to store the developed color image.
In view of these findings, we have carried out further studies. As a result, we have now found that the problems described above can be solved at one stroke by using a heat transferable sheet which has a specific structure. The present invention has been developed on the basis of this discovery.